Coriolis mass flow meter with thick wall measuring tube

ABSTRACT

A mass flow meter for flowing media, which operates according to the Coriolis principle, has a straight Coriolis measuring tube, an oscillation generator acting on the Coriolis measuring tube, and at least one sensor detecting Coriolis forces and/or Coriolis oscillations based on Coriolis forces. The problem which results from the fact the mass flow meter has only one straight Coriolis measuring tube is largely eliminated by designing the Coriolis measuring tube as a flow channel of a thick-walled body, namely a thick-walled tube, the thick-walled tube having recesses accessible from the outside and reaching very close to the Coriolis measuring tube. A vibration transducer as the oscillation generator acts upon the residual material of the thick-walled tube remaining in the area of the recesses, and the Coriolis forces or Coriolis oscillations appearing in the area of the residual material of the thick-walled tube are detected by each sensor.

RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 09/219,405, filedDec. 23, 1998 now abandoned, which is a continuation of Ser. No.08/981,938, filed Dec. 30, 1997 now U.S. Pat. No. 5,918,285.

The invention concerns a mass flow meter for flowing media, whichoperates according to the Coriolis principle, with one at leastessentially straight Coriolis measuring tube, with at least oneoscillation generator acting on the Coriolis measuring tube, and with atleast one sensor detecting Coriolis forces and/or Coriolis oscillationsbased on Coriolis forces.

BACKGROUND OF THE INVENTION

Mass flow meters for flowing media, which operate according to theCoriolis principle, are known in different designs (cf. e.g. GermanPatent Specification 41 24 295 and German Offenlegungsschrift 41 43 361and the publications cited there in each case in column 1, lines 20 to27, German Patent Specification 42 24 397 and the publications citedthere in column 1, lines 23 to 30, as well as German Offenlegungsschrift196 01 342) and have been increasingly used in practice for some time.

In the case of mass flow meters for flowing media, which operateaccording to the Coriolis principle, basically, there are, on the onehand, those whose Coriolis measuring tube is made at least essentiallystraight, as a rule exactly straight, and, on the other hand, thosewhose Coriolis measuring tube is made loop-shaped. In addition, in thecase of the mass flow meters under discussion, there are, on the onehand, those which have only one Coriolis measuring tube and, on theother hand, those which have two Coriolis measuring tubes. In the caseof the embodiments with two Coriolis measuring tubes, they can behydraulically in series or parallel to one another.

Mass flow meters of the type in question, in the case of which theCoriolis measuring tube is, or the Coriolis measuring tubes are, madestraight, with respect to their mechanical construction are simple andcan consequently be produced at relatively low cost. In this case, it isalso possible to finish or polish the inner surfaces of the Coriolismeasuring tube or Coriolis measuring tubes well; they can be polishedeasily. In addition, they have a relatively low pressure loss. In thecase of mass flow meters which operate according to the Coriolisprinciple, and whose Coriolis measuring tube is made straight, or theCoriolis measuring tubes are made straight, it can be disadvantageousthat both thermally caused expansions or stresses, as well as forces andmoments acting from outside, can lead to measurement errors and tomechanical damage, namely stress cracks.

The experts have already dealt with the above-mentioned problems in massflow meters with straight Coriolis measuring tubes (cf. in particularGerman Patent Specification 41 24 295, German Offenlegungsschrift 41 43361, and German Patent Specification 42 24 379). Altogether, a mass flowmeter operating according to the Coriolis principle, with a straightCoriolis measuring tube, which has a measurement error of only about0.1% (cf. the prospectus “Zulassung des Corimass G-Gerätes zumeichpflichtigen Verkehr” of the KROHNE Meβtechnik GmbH & Co. KG), wasmade successfully.

Mass flow meters operating according to the Coriolis principle, whichhave only one straight Coriolis measuring tube, have considerableadvantages as compared with those mass flow meters which have either twostraight Coriolis measuring tubes or one loop-shaped Coriolis measuringtube. The advantage as compared with mass flow meters with two straightCoriolis measuring tubes in particular is to be seen in the fact thatflow separators or flow combiners, which are required in the case ofmass flow meters with two Coriolis measuring tubes, are not needed. Theadvantage as compared with flow meters with one loop-shaped Coriolismeasuring tube, or with two loop-shaped Coriolis measuring tubes, inparticular is to be seen in the fact that a straight Coriolis measuringtube is easier to produce than a loop-shaped Coriolis measuring tube,that the pressure drop in the case of a straight Coriolis measuring tubeis less than in the case of a loop-shaped Coriolis measuring tube, andthat a straight Coriolis measuring tube can be cleaned better than aloop-shaped Coriolis measuring tube.

However, mass flow meters which operate according to the Coriolisprinciple and have one straight Coriolis measuring tube, also have aphysically, or mechanically, predetermined disadvantage (cf. EuropeanOffenlegungsschrift 0 521 439).

Mass flow meters operating according to the Coriolis principle requirethat the Coriolis measuring tube be put into oscillation by means of atleast one oscillation generator; the Coriolis forces, or the Coriolisoscillations, do indeed result from the fact that the Coriolis measuringtube oscillates and from the flowing of mass through the Coriolismeasuring tube.

In the case of mass flow meters with two straight Coriolis measuringtubes, or with one loop-shaped Coriolis measuring tube, or with twoloop-shaped Coriolis measuring tubes, the Coriolis measuring tubes, orthe parts of the loop-shaped Coriolis measuring tubes causingoscillation, are designed identically and located and excited intooscillation so that they oscillate opposite one another. This has thepositive consequence that the oscillating system as a whole is notacting as such outwards. The position of the center of mass remainsconstant and forces which appear are compensated. Consequently, nooscillations are introduced into the pipeline system in which this massflow meter is installed, and oscillations of the pipeline system do notinfluence the measurement result.

In the case of mass flow meters operating according to the Coriolisprinciple, which have only one straight Coriolis measuring tube, thepositive consequence of Coriolis measuring tubes oscillating oppositeone another, explained above, naturally does not occur. The center ofmass does not remain constant and forces which appear are notcompensated. The consequence of this is, on the one hand, thatoscillations are transferred into the pipeline system in which a massflow meter is installed, and on the other hand, that oscillations of thepipeline system can also influence the measurement result.

SUMMARY OF THE INVENTION

The object of the invention now is to provide a mass flow meteroperating according to the Coriolis principle, in the case of which theproblem, previously discussed in detail, which results from the factthat the mass flow meter has only one straight Coriolis measuring tube,is of less consequence.

The mass flow meter in accordance with the invention, in the case ofwhich the problem derived and presented previously in detail is solved,now in the first place and essentially is characterized by the fact thatthe Coriolis measuring tube is designed as a flow channel of athick-walled body, in particular a thick-walled tube, that thethick-walled tube has recesses accessible from the outside, reachingvery close to the Coriolis measuring tube, that the oscillationgenerator acts upon the residual material of the thick-walled tubesremaining in the area of the recesses, and that the Coriolis forces orCoriolis oscillations appearing in the area of the residual material ofthe thick-walled tube are detected by the sensor or the sensors.

In the case of the mass flow meter in accordance with the invention, themass of the residual material of the thick-walled tube acting as aCoriolis measuring tube as a whole is relatively small in relation tothe mass of the thick-walled tube. From this it results that the “centerof mass not constant” problem discussed initially does remainqualitatively, but quantitatively has practically no effect. This may beseen as a summary of the advantages achieved by the invention.

In particular, there are now a number of possibilities for designing andfurther developing the mass flow meter in accordance with the invention.We refer, on the one hand, to the patent claims, and, on the other hand,to the description of the preferred embodiments in connection with thedrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a longitudinal section through a first embodiment of a massflow meter in accordance with the invention,

FIG. 2 shows a longitudinal section through a second embodiment of amass flow meter in accordance with the invention,

FIG. 3 shows a longitudinal section through a third embodiment of a massflow meter in accordance with the invention,

FIG. 4 shows a longitudinal section through a fourth embodiment of amass flow meter in accordance with the invention,

FIG. 5 shows a longitudinal section through a fifth embodiment of a massflow meter in accordance with the invention,

FIG. 6 shows a longitudinal section through a sixth embodiment of a massflow meter in accordance with the invention,

FIG. 7 shows a longitudinal section through a seventh embodiment of amass flow meter in accordance with the invention,

FIGS. 8a to 8 c show a longitudinal section through an eighth embodimentof a mass flow meter in accordance with the invention,

FIG 9. shows a possible cross-section of a mass flow meter in accordancewith the invention,

FIG. 10 shows a further possible cross-section of a mass flow meter inaccordance with the invention,

FIG. 11 again shows a possible cross-section of a mass flow meter inaccordance with the invention,

FIG. 12 shows a cross-section through an entirely different embodimentof a mass flow meter in accordance with the invention, and

FIG. 13 shows a schematic representation for explaining another entirelydifferent embodiment of a mass flow meter in accordance with theinvention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

The mass flow meter for flowing media in accordance with the inventionis one which operates according to the Coriolis principle. In the firstplace, as a rule, but not necessarily, the mass flow meter in accordancewith the invention has a housing not shown in the figures. An at leastessentially, as a rule and in the embodiments shown exactly, straightCoriolis measuring tube 1, at least one oscillation generator 2 actingon the Coriolis measuring tube 1, and at least one sensor 3, as a ruletwo sensors 3, detecting Coriolis forces and/or Coriolis oscillationsbased on Coriolis forces are functionally necessary to the mass flowmeter in accordance with the invention.

As FIGS. 1 to 5 and 7 to 11 show, the Coriolis measuring tube 1 is madeas a flow channel of a thick-walled body, namely a thick-walled tube 4.The thick-walled tube 4 has recesses 5 accessible from outside andreaching close to the Coriolis measuring tube 1. In this way, onlyresidual material 6 remains in the areas of the recesses 5 of thethick-walled tube 4; therefore the otherwise thick-walled tube 4 has—atleast partially—a relatively small wall thickness in the area of therecesses 5. The oscillation generator 2 acts on the residual material 6of the otherwise thick-walled tube 4 remaining in the area of therecesses 5. This leads to the fact that the relatively thin-walledresidual material 6 of the otherwise thick-walled tube 4 acting as theactual Coriolis measuring tube 1 is made to oscillate, so that Coriolisforces and resulting Coriolis oscillations appear in connection with themass of a flowing medium. The Coriolis forces, or the Coriolisoscillations based on these Coriolis forces, appearing in the area ofthe residual material 6 of the otherwise thick-walled tube 4 aredetected by each sensor 3 and then evaluated in the usual way.

In the case of the mass flow meter in accordance with the invention, thethick-walled tube 4 can consist of metal, or of a metal alloy, or ofplastic, in particular of polytetrafluorethylene (PTFE), ofperfluouro-alkoxy-polymer (PFA), or of polyether etherketone (PEEK).

With respect to the geometry of the Coriolis measuring tube 1, and withrespect to the geometry of the thick-walled tube 4, the designercarrying out the theory of the invention has considerable freedom herein particular. Both the Coriolis measuring tube 1 as well as thethick-walled tube 4 can have a round, an oval, an elliptical, or arectangular, in particular square, cross-section. FIGS. 9, 10, and 11show only as examples different cross-sectional shapes with respect tothe Coriolis measuring tube 1 and the thick-walled tube 4.

In the case of mass flow meters which operate according to the Coriolisprinciple, thus also in the case of the mass flow meter in accordancewith the invention, thermal influences can affect the measurementsensitivity and/or the zero point—and thus the measurement accuracy ofthe flow meter as a whole. Consequently, there is a proposal to provideat least one temperature sensor T (FIG. 7) in the flow meter, in orderto compensate for thermal influences on the measurement sensitivityand/or on the zero point of the flow meter.

Initially, it was stated that mass flow meters of the type underconsideration, therefore also the mass flow meter in accordance with theinvention, include at least one oscillation generator 2 acting on theCoriolis measuring tube 1. However, preferably, as also is shownpartially in the drawing figures, two oscillation generators 2 areprovided and the two oscillation generators 2 are arranged symmetricalto the longitudinal axis of the Coriolis measuring tube 1, respectivelyto the thick-walled tube 4. In this case, the oscillation generators 2can be operated in phase or in phase opposition.

There follows the details of the embodiments of mass flow meters inaccordance with the invention shown in FIGS. 1 to 8.

The theory embodied in the mass flow meter in accordance with theinvention, as discussed previously, essentially consists in the factthat the Coriolis measuring tube 1 is made as a flow channel of athick-walled body, in particular a thick-walled tube 4, and that thethick-walled tube 4 has recesses 5, accessible from outside, reachingrelatively close to the Coriolis measuring tube 1, so that onlyrelatively thin-walled residual material 6 remains in the area of therecesses 5. The recesses 5 provided in accordance with the inventioncan, as is shown in FIG. 5, be realized on one side on the initiallythick-walled tube 4, as is shown in FIGS. 1 to 4 and 6 to 8. However,there is also the possibility, as is shown in FIG. 5, of making therecesses 5 on both sides in the otherwise thick-walled tube 4. This isrequired when, as is also shown in FIG. 5, two oscillation generators 2are provided and the oscillation generators 2 are to be locatedsymmetrical to the longitudinal axis of the Coriolis measuring tube 1 orof the thick-walled tube 4.

Moreover, in the mass flow meter in accordance with the invention, theoscillation generator 2 (or the two oscillation generators 2) and thesensor(s) 3 can be designed completely differently.

In the embodiment shown in FIG. 1, the oscillation generator 2 and thesensors 3 are designed conventionally in the broadest sense. In the caseof the embodiment shown in FIG. 2, the oscillation generator 2 and thesensors 3 in connection with a common carrier 7 are made out ofpiezo-material, separate layers for the oscillation generator 2 and thesensors 3 being provided. The embodiment shown in FIG. 3 concerns amagneto-restrictive oscillation generator 2, while the sensors 3 consistof piezo-material and they project out of the recesses 5 of thethick-walled tube 4, so that the sensors 3 are largely decoupled fromthe temperature of the thick-walled tube 4. FIG. 4 shows an embodimentwhose oscillation generator 2 is operated in longitudinal resonance,while the sensors 3 are magneto-dynamic sensors. The embodiment shown inFIG. 5, as already discussed, has two oscillation generators 2 which canbe operated either in phase or in phase opposition. It is to be notedhere that basically it is also possible to have embodiments of mass flowmeters in accordance with the invention, in the case of which one andthe same component acts both as an oscillation generator 2 and as asensor 3. However, in the embodiment shown in FIG. 5, the sensors 3 aredesigned as ultrasonic transmitters and receivers. The ultrasonic wavesemitted by the sensors 3 are introduced into the Coriolis measuring tube1 and reflected back through the inner wall of the Coriolis measuringtube 1—through an acoustic impedance step change—and received by thesensors 3 again. The Coriolis forces or the Coriolis oscillationsresulting therefrom acting on the Coriolis measuring tube 1 can beevaluated by interference measurement. In the case of this embodimentthe thick-walled tube 4 preferably consists of a material with a lowmodulus of elasticity E. In the embodiment shown in FIG. 6, it is madeclear that there are two oscillation generators 2 which are located andmechanically coupled on both sides of the thick-walled tube 4. Theembodiment shown in FIG. 7 provides a magnetic oscillation generator 2which excites the Coriolis measuring tube 1, therefore the residualmaterial 6 of the otherwise thick-walled tube 4, in the bending mode.Moreover, FIG. 7 shows an embodiment in the case of which three sensors3 are provided, it being possible to use the middle sensor 3 forcorrection or compensation purposes.

FIGS. 8a, 8 b, and 8 c, show an embodiment of the mass flow meter inaccordance with the invention, in which it is shown that differentexcitation modes can be realized. One excitation mode is indicated inFIG. 8a, another in FIG. 8b. FIG. 8c shows a concrete embodiment forrealizing the excitation mode shown in FIG. 8b. In the case of thisembodiment, the oscillation generator 2 is designed to be magneticallyinductive. It is immediately clear that this embodiment and applicationof the oscillation generator 2 lead to the excitation mode shown in FIG.8b.

It has been explained above that, in the case of the mass flow meter inaccordance with the invention, the mass of the residual material 6 ofthe thick-walled tube 4 acting as a Coriolis measuring tube 1 isrelatively small in relation to the mass of the thick-walled tube 4 as awhole, and that from this it results that the “center of mass notconstant” problem explained initially does remain qualitatively, butquantitatively has practically no effect when the Coriolisoscillations—in the case of a thick-walled tube 4—are decidedly small.This is attainable when, according to a further embodiment of theinvention oscillation, generators in the form of vibration converters 8are provided on the outside of the thick-walled tube 4 and the vibrationfrequency is chosen so that a maximum deflection of the inner wall ofthe thick-walled tube 4 takes place. FIGS. 12 and 13 show embodiments ofthe previously described mass flow meter in accordance with theinvention, in the case of which the recesses provided in the case of theembodiments of the mass flow meters in accordance with the inventiondescribed above are not realized. High cross-currents to the directionof flow, which in connection with the mass of the flowing medium lead torelatively great Coriolis forces, the reaction of which on the innerwall of the thick-walled tube 4 can be detected, appear as a result ofthe theory of the invention described previously.

Finally, it can be advantageous to adapt the sound frequency to theflowing medium—by control or regulation—so that even-numbered multiplesor even-numbered fractions of the wave length in the flowing mediumcorrespond at least approximately to the inner diameter of the Coriolismeasuring tube 1 or the distance from one inner wall to the oppositeinner wall of the Coriolis measuring tube 1.

The oscillation generators in the form of vibration converters 8, i.e.,vibration transducers, are preferably provided on the outside of thethick-walled tube 4 acting on the Coriolis measuring tube 1 opposite toeach other. The thick-walled tube 4 is exited by the vibrationconverters 8 such that a maximum deflection of the inner wall of theinner surface of the thick-walled tube 4 takes place.

A typical vibration converter or vibration transducer is comprised of anelectrical coil with an axially movable ferromagnetic core. When an ACcurrent is applied to the coil, the core performs an harmonicoscillation relative to the coil.

The effect which is achieved with such an arrangement is the following.The vibration converters provided on the outside of the thick-walledtube generate pressure waves which propagate radially through the wallof the Coriolis measuring tube as variations of density of the materialof the tube. At the interface between the media flowing through the tubeand the media of the Coriolis measuring tube, these density waves arepartly reflected. The waves generated by the vibration converter and thereflected wave have the same frequency and the same amplitude. Two waveswith the same frequency and the same amplitude propagating in oppositedirections towards each other generate a standing wave. This standingwave has oscillation nodes with a distance from each other of half thewavelength, and between these nodes antinodal points with maximumvibration. On both sides of the nodes, the particles vibrate in phasesuch that the media in the nodes is alternately compressed and dilated,respectively. Nodes of the velocity of the movement of the particlesrefer to antinodes of the pressure or density. The pressure wave and thewave of the deflection of the particles have a phase different of π.

At the position where the vibration converter is fixed, a pressure node,i.e., a maximum deflection is generated. In order to achieve a maximumdeflection of the inner surface of the thick-walled tube, a pressurenode has to be generated at this surface too. Thus, for achieving amaximum deflection of the inner surface of the thick-walled tube, thevibration frequency of the vibration converter has to be chosen suchthat the thickness of the thick-walled tube has to be an integralmultiple of half the wavelength of the pressure wave propagating throughthe wall of the tube.

What is claimed is:
 1. A mass flow meter for flowing media, whichoperates according to the Coriolis principle, with one at leastessentially straight Coriolis measuring tube having a thick wall with anoutside and an inside surface defining a flow channel with at least onevibration converter acting on the Coriolis measuring tube, so that apressure wave is propagated through the wall of the tube, said pressurewave having a wavelength, and with at least one sensor detecting atleast one of Coriolis forces and Coriolis oscillations based on Coriolisforces, wherein said vibration converter is on the outside of theCoriolis measuring tube and the vibration frequency generated by thevibration converter is such that the thickness of said tube wall is anintegral multiple of half the wavelength of the pressure wavepropagating through the tube wall whereby a maximum deflection of saidinner surface takes place.
 2. A mass flow meter in accordance with claim1, wherein the Coriolis measuring tube consists of metal or of a metalalloy.
 3. The mass flow meter in accordance with claim 1, wherein theCoriolis measuring tube consists of plastic, in particular ofpolytetrafluorethylene (PTFE), of perfluoroalkoxy-polymer (PFA), or ofpolyetheretherketone (PEEK).
 4. The mass flow meter in accordance withany one of the claims 1 to 3, wherein said Coriolis measuring tube has around, an oval, an elliptical, or a rectangular, in particular a square,cross-section.
 5. The mass flow meter in accordance with any one of theclaims 1 to 3 wherein the outside of said thick wall has a round, oroval, an elliptical, or a rectangular, in particular a square,cross-section.
 6. The mass flow meter in accordance with any one of theclaims 1 to 3, wherein at least one temperature sensor is provided forcompensation of thermal influences on the measurement accuracy and thezero point of the mass flow meter.
 7. The mass flow meter in accordancewith any one of the claims 1 to 3, wherein two oscillation generatorsare provided and both oscillation generators are arranged symmetrical tothe longitudinal axis of the Coriolis measuring tube.
 8. The mass flowmeter in accordance with claim 7, wherein the oscillation generators areoperated in phase.
 9. The mass flow meter in accordance with claim 7,wherein the oscillation generators are operated in phase opposition. 10.The mass flow meter in accordance with any one of the claims 1 to 3,wherein the sound frequency is adapted to the flowing medium—by controlor regulation—so that even-numbered multiples or even-numbered fractionsof the wave length in the flowing medium correspond at leastapproximately to the inner diameter of the Coriolis measuring tube orthe distance from one inner wall to the opposite inner wall of theCoriolis measuring tube.
 11. The mass flow meter in accordance with anyone of claims 1-3 wherein at least one temperature sensor is providedfor compensattion of thermal influences on the measurement accuracy orthe zero point of the mass flow meter.
 12. A mass flow meter for flowingmedia which operates according to the Coriolis principle with one atleast essentially straight Coriolis measuring tube having a thick wallwith an outside and an inner surface defining a flow channel, with atleast one sensor detecting at least one of Coriolis forces and Coriolisoscillations based on Coriolis forces, and with two vibration convertersbeing provided on the outside of the Coriolis measuring tube opposite toeach other and generating vibrations so that a pressure wave ispropagated through the tube wall, said pressure wave having awavelength, the frequencies of said vibrations being such that thethickness of said tube wall is an integral multiple of half thewavelength of said pressure wave whereby maximum deflection of saidinner surface takes place.
 13. A mass flow meter in accordance withclaim 12, wherein the Coriolis measuring tube consists of metal or of ametal alloy.
 14. The mass flow meter in accordance with claim 12,wherein the Coriolis measuring tube consists of plastic, in particularof polytetrafluorethylene (PTFE), of perfluoroalkoxy-polymer (PFA), orof polyetherketone (PEEK).
 15. The mass flow meter in accordance withclaim 12, wherein said Coriolis measuring tube has a round, an oval, anelliptical, or a rectangular, in particular a square, cross-section. 16.The mass flow meter in accordance with claim 12, wherein at least onetemperature sensor is provided for compensation of thermal influences onthe measurement accuracy and the zero point of the mass flow meter. 17.The mass flow meter in accordance with claim 12 wherein both vibrationgenerators are arranged symmetrical to the longitudinal axis of theCoriolis measuring tube.
 18. The mass flow meter in accordance withclaim 17 wherein the vibration generators are operated in phase.
 19. Themass flow meter in accordance with claim 17 wherein the vibrationgenerators are operated in phase operation.
 20. The mass flow meter inaccordance with claim 12 wherein the sound frequency is adapted to theflowing medium—by control or regulation—so that even-numbered multiplesor even-numbered fractions of the wavelength in the flowing mediumcorrespond at least approximately to the inner diameter of the Coriolismeasuring tube or the distance from one inner wall to the opposite innerwall of the Coriolis measuring tube.
 21. The mass flow meter inaccordance with claim 12 wherein at least one temperature sensor isprovided for compensation of thermal influences of the measurementaccuracy or the zero point of the mass flow meter.